scanners and "cell blocked" ?

[Rodo]

Hi all,

I've been looking for a scanner to do some experiments and I got confused by
the "Cell blocked" fine print. I don't really care to listen to cell phone
conversations of any kind. I thought most cell modulations are either
digital or code hoping or both. Only old cell phones could potentially be
heard on a scanner. A professor in college said that cell phones frequency
hop every 16 ms ( or so ). This is old info (hopefully reliable). So, I
imagine the new stuff is really protected from a simple scanner ? Yet, all
scanners sold in the USA are "cell blocked". Could someone shed some info on
the subject ?

Thanks

 

[Ken Taylor]

"Rodo" <dsp1024@yahoo.com> wrote in message
news:d4Vae.3626$yc.3477@trnddc04...

Hi all,

I've been looking for a scanner to do some experiments and I got confused
by
the "Cell blocked" fine print. I don't really care to listen to cell phone
conversations of any kind. I thought most cell modulations are either
digital or code hoping or both. Only old cell phones could potentially be
heard on a scanner. A professor in college said that cell phones frequency
hop every 16 ms ( or so ). This is old info (hopefully reliable). So, I
imagine the new stuff is really protected from a simple scanner ? Yet, all
scanners sold in the USA are "cell blocked". Could someone shed some info
on
the subject ?

Thanks


"Cell blocking" dates back to the old analog systems, which are rare now but

still used in places. Modern digital systems are not able to be eavesdropped
on by the casual listener. AFAIK only the US mandates blocking of the
frequency range commonly used by cell systems, so any coming from overseas
could tune up to listen to white noise or other undecipherable garbage.

A CDMA system may hop frequency as often as you say, but others (GSM,
D-AMPS, etc) don't. Their encoding algorithms provide enough content
protection for scanners to be useless against them. CDMA doesn't do the
hopping just to make them 'invisible' to scanners, of course, but that's a
nice side effect.

Ken

 

[Jonathan Westhues]

"Ken Taylor" <ken@home.nz> wrote in message
news:yCVae.1100$Od6.164221@news.xtra.co.nz...

A CDMA system may hop frequency as often as you say, but others (GSM,
D-AMPS, etc) don't. Their encoding algorithms provide enough content
protection for scanners to be useless against them. CDMA doesn't do the
hopping just to make them 'invisible' to scanners, of course, but that's a
nice side effect.

GSM may or may not frequency-hop, depending on the network settings. GSM
uses a narrow-band modulation scheme (GMSK) but if frequency hopping is
enabled then it will use a different centre frequency for each burst. Error
correcting codes are used to spread the information in a given block over
multiple bursts, so that if a single frequency happens to be faded then you
can still recreate the block.

For GSM at least, there is even some encryption on top of that, so that the
restriction is particularly stupid. See the FCC Part 15 regulations section
15.121 though.

Jonathan
http://cq.cx/